Page 34 - ITU Journal Future and evolving technologies Volume 2 (2021), Issue 7 – Terahertz communications
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ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 7




         10 −12 ) [12]. The authors also estimated that their system   In  general,  this  increase  in  spectral  ef iciency  makes
          could extend to span even farther ranges with the use of   higher order modulations the most attractive, due to the
          more advanced error‑correction codes.                fact that more information can be sent within in a given
                                                               bandwidth  or,  conversely,  that  the  system  requires  less
          Collectively,  these  demonstrations  indicate  that  long‑   bandwidth to maintain a given data rate.
         range terahertz communication links are not only possi‑
         ble,  but  will  likely  be  implemented  commercially  in  the
         foreseeable  future  as  the  technology  progresses.  This   However, higher‑order modulations are not always viable
         technology would provide many bene its, since there are   to  use.  When  a  system  is  constrained  to  operate  below
         many  situations  in  which  the  ability  to  rapidly  estab‑   some   ixed  maximum  power,  the  phase  and  amplitude
                                                               of  all  communication  symbols  must  fall  within  a   inite
         lish  a  directive,  wireless  point‑to‑point  link  with  a  ca‑
                                                               region  of  the  phase/amplitude  plane  that  satis ies  that
         pacity of tens of Gb/s would be highly attractive,  inclu-
                                                               power constraint.  Increasing the number of communica‑
         ding  temporary  installments  during  disaster  recovery
         or  wartime  environments,  at  locations  where  trenching   tion  symbols  necessarily  means  that  symbols  must  take
          iber is prohibitively expensive or time‑consuming, or as   on increasingly similar values of amplitude and/or phase,
         a  re‑  placement  to  upgrade  microwave  point‑to‑point   as  more  symbols  have  to  be  placed  within  the   inite  re‑
         backhaul links.                                       gion  satisfying  the  power  constraint.  When  the  receiver
                                                               must  differentiate  between  a  large  number  of  similar
         There are many design choices that must be considered
                                                               symbols  with  high  resolution,  injected  noise  can  easily
         when planning the construction of such a link [13, 14, 15,
                                                               shift  the  amplitude  and/or  phase  of  the  received
         16].  One  notable  known  design  choice  is  selecting  the
                                                               waveform  so  that  symbols  are  received  in  error,  much
         modulation  scheme,  since  modulation  type  determines
                                                               more so than for a lower‑order modulation scheme where
         the  shape  of  the  temporal  waveform,  the  hardware  re‑
                                                               symbol regions are  larger  and  more  widely  spaced.  As
         quirements (for example, the dynamic range of the front‑
                                                               a  result,  higher‑order  M‑QAM  schemes  have  more
         end receiver), the resilience of the channel to interference,
                                                               stringent requirements on the minimum SNR allowed at
         and  the  achievable  throughput  of  the  channel.  By  judi‑
                                                               the receiver for effective operation.  Fig.  1  illustrates  the
         cious selection of the modulation type, channel through‑
                                                               increase  in  SNR  required  by  higher‑order  M‑QAM
         put  can  be  maximized,  and  many  research  teams  have
                                                               schemes  in  order  to  maintain  a given bit error rate.
         investigated  various  algorithms  and  strategies  for
         determining  the  optimal  modulations  for  both
         microwave and terahertz wireless links [17, 18, 19].  When  designing  a  communication  link,  the  modulation
                                                                type  is  chosen  so  that  an  acceptable  error  rate  is  main‑
          Many modulation schemes are possible, and all carry their
                                                                tained  under  the  worst‑case  SNR  the  link  is  designed  to
          own    its  and  drawbacks.  However,  the  prototype
                                                                handle.  In  order  to  decrease  the  outage  probability  du-
          terahertz  links  in  the  demonstrations  listed  earlier  em‑ ploy
                                                                ring  times  when  the  signal  is  strongly  attenuated,  and
          various  orders  of  quadrature  amplitude  modulation,
                                                                to  increase  the  capacity  when  channel  conditions  are
          collectively  known  as  M‑QAM  schemes,  including  Binary
                                                                favorable,   many   communication   systems   employ
          Phase  Shift  Keying  (BPSK,  or  2‑QAM),  Quadrature  Phase
                                                                optimization  algorithms  that  actively  select  the  order  of
          Shift  Keying  (QPSK,  or  4‑QAM),  8‑QAM  and  16‑QAM.  In an
                                                                the modulation  scheme  used  [17].  These  optimization
          M‑QAM scheme, binary data is encoded as communi‑ cation
                                                                routines  switch between  modulation  orders  as  channel
          symbols,  distinct  combinations  of  amplitude  and  phase  of
                                                                conditions  vary, such that the resulting link is both more
          the  carrier  wave,  each  of  which  represent  one or more bits
                                                                reliable  (in  terms  of  outage  probability)  and  operates
          of data.  The modulation order M  speci ies how many such
                                                                with a higher average capacity.
          combinations of amplitude and phase are recognized by the
          receiver,  and  log (  )  bits  of  data  are  carried  by  each
                        2
          symbol.
                                                               Terahertz links will, of course, likewise bene it from these
         As  the  modulation  order  of  the  communication  system is   type of optimization routines [18, 17], whether the band‑
         increased,  each  symbol  transition  carries  more  infor‑   width  is  occupied  by  a  single  link,  or   illed  with  a  large
         mation,  which  consequently  increases  the  spectral  ef i‑   number  of  subcarriers  [21].  However,  due  to  the  huge
         ciency of the link. Spectral ef iciency is a measure of how   bandwidths available for terahertz communication links,
         many bits of data are transferred per unit of bandwidth   and the high frequencies at which they operate, the opti‑
         utilized by the communication system, typically given in   mal modulation type will not be determined by SNR (that
         units of  (   bits )                                  is, fading) alone.  Our work indicates that the Group Velo-
                      / Hz. While the spectral ef iciency realized in a
         physical communication system depends on many factors   city  Dispersion  (GVD)  caused  by  molecular  resonances
         (such as the coding scheme, Signal‑To‑Noise Ratio (SNR),   in  the  atmosphere  can  result  in  counter‑intuitive
         and fading characteristics of the channel), the theoretical   behavior  over  the  lower  terahertz  bands,  in  which  the
         maximum spectral ef iciency of an M‑QAM scheme is ulti‑   severity  of  Inter‑Symbol  Interference  (ISI)  depends  not
         mately  given  by,  and  scales  with  modulation  order   only  on  band‑  width  (as  expected),  but  also  on  the
                                                               modulation type used, even in the absence of noise.
         according to, log2(  ) [20].
          22                                 © International Telecommunication Union, 2021
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